Show only one log warning message about missing icon for a file extension.

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Describe the bug
When the application fails to find an icon based on a file's extension, the warning message is displayed multiple times in the logs:

W0924 17:29:09.709913  7076 Icon.cpp:93] Failed to find icon for file extension 'md'. Resolving icon with default icon for unknown file type 'C:\Windows\system32\imageres.dll' with index '2'
W0924 17:29:13.807147  7076 Icon.cpp:93] Failed to find icon for file extension 'md'. Resolving icon with default icon for unknown file type 'C:\Windows\system32\imageres.dll' with index '2'
W0924 17:30:29.756491  7076 Icon.cpp:93] Failed to find icon for file extension 'md'. Resolving icon with default icon for unknown file type 'C:\Windows\system32\imageres.dll' with index '2'
W0924 17:30:40.330096  7076 Icon.cpp:93] Failed to find icon for file extension 'md'. Resolving icon with default icon for unknown file type 'C:\Windows\system32\imageres.dll' with index '2'
W0924 17:30:45.859412  7076 Icon.cpp:93] Failed to find icon for file extension 'md'. Resolving icon with default icon for unknown file type 'C:\Windows\system32\imageres.dll' with index '2'
W0924 21:01:27.486472  7076 Icon.cpp:93] Failed to find icon for file extension 'md'. Resolving icon with default icon for unknown file type 'C:\Windows\system32\imageres.dll' with index '2'
W0924 21:04:31.776013  7076 Icon.cpp:93] Failed to find icon for file extension 'md'. Resolving icon with default icon for unknown file type 'C:\Windows\system32\imageres.dll' with index '2'
W0924 21:04:48.452966  7076 Icon.cpp:93] Failed to find icon for file extension 'md'. Resolving icon with default icon for unknown file type 'C:\Windows\system32\imageres.dll' with index '2'

This log message appears every time the system cannot find the icon for md files. This message is not required to be printed multiple times.

To Reproduce
Steps to reproduce the behavior:

1. Create a file with an unknown extension. For example test.12345.
2. Create a Configuration which includes a menu with an <icon> using the file's extension. For example: <icon fileextension="12345" />
3. Right-click on the file to show the menu.
4. Look at the WARNING level logs.

Expected behavior
The message should appears only once per file extension.

Screenshots
N/A

Environment

• OS: Windows 7
• Version 0.6.0

Additional context
Add any other context about the problem here.

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There is also another problem related to this issue when multiple files are selected.

In the default.xml configuration file, the menu "Copy to clipboard, Full Path" uses <icon fileextension="${selection.filename.extension}" /> which creates an invalid request if multiple files are selected.

For example, if the user selects multiple files, the following is shown in the WARNING level log:

W1111 09:30:43.839697 27764 Icon.cpp:93] Failed to find icon for file extension 'xml
jpg
mp3
exe
dll
mp4'. Resolving icon with default icon for unknown file type 'C:\Windows\system32\imageres.dll' with index '2'

The implementation should detect when multiple files are selected by reading property selection.multi.separator. If multiple file extensions are selected, try to resolve the icon with the first extension. Based on the example above, the icon should be resolved from the xml file extension. Of course, if resolving the icon for xml has failed before, no error message should be displayed in the WARNING log.