Added a taskbar squash option

[NoakPalander]

Haven't found any existing options that can achieve this, if I'm incorrect then please point me out to how. Otherwise:

Added a squash-list option to wlr/taskbar that takes titles or app_id's, when set then at most 1 icon can be shown of the same app id. E.g if 15 Firefox instances is opened and "squash-list": ["firefox"] is set, then only 1 Firefox icon is displayed. The value * can be used to make it apply to all title/ids.

This also doesn't interfere with ignore-list that takes higher priority.

When the only visible instance of an application is killed, assuming there's more instances open, it finds any suitable one and shows that task's button instead in order to maintain one icon per application.

[sjoblomj]

I like the idea of this, but it seems a little bit buggy:

• If I click three times on a squashed application, it vanishes.
• If I use "squash-list": ["*"], I see no applications at all in the task bar.