Sui(change): Use libappindicator

[ShadowRZ]

This would use libappindicator to implment system tray.

Tested on:

• [ ] GNOME (With appindicator extension)
• [x] KDE

Functionality:

• [ ] ~~Click to hide/show Srain windows~~ Looks like AppIndicator doesn't support this (No active event or mouse event)
• [x] Menu:
  • [x] Preferences
  • [x] About
  • [x] Exit
• [x] Icon (Normal / Highlight)

Note: Maybe AppIndicator doesn't support reading icons from gresources, so I have to change data/Makefile to install the attention icon too.

[SilverRainZ]

Click to hide/show Srain windows

I think it is an important function.

Beside, does appindicator works on windows?

[SilverRainZ]

Sigh, the latest version of appindicator is released on 2012-07-11.

[SilverRainZ]

As we are migrating to GTK4, this PR will be merge at that time.

[SilverRainZ]

This PR has conflicts and I can't rebase your branch, so close.

I will open a new one.