Filter playlist items

[pietrocaselani]

This pull request introduces a text box in the playlist panel to filter playlist items whose "label" matches the text.

Here is a screenshot of what it looks like:

I was unable to match the text box style to that of the application.
Any feedback on how to improve this PR will be welcome.

Related to issues: #168 and #3748

[bibsp]

1. Text size and scaling is not perfect. From the screenshot I can see.
2. Complete white background is also something that doesn't look good in dark theme.

[pietrocaselani]

Thanks for the feedback @bibsp

I will update this pull request and look for examples in the code to adjust the text styling, font, scaling, and the theme.

[adipose]

CMPCThemeEdit

[clsid2]

Does this start filtering directly after first key press (like letter 'a')? Maybe better to require a minimum length of 3 before first filtering action. And press Enter for shorter strings.

This requires a lot of testing of various situations where the contents of the playlist gets modified.

What happens if filter is active while playing and a transition happens to a next file in playlist. Does that break anything? And after clearing search filter, does the correct active playlist item get selected?

What happens if there is single item in playlist (current file). I do a search. Then press PageDown to go to next file in folder, which replaces playlist.

Open a new playlist file while a filter is being applied. Does it correctly load new playlist? Does it apply filter again afterwards?

etc.