gs-application: Add tooltips

Disclaimer: I know very little about C.

This MR adds a tooltip for the following:

• Search
• "Main Menu" (Hamburger menu)
• "Remove" (to remove a repository from the Software Repositories dialog)
• "Refresh" (to refresh updates)
• "Stop Refreshing" (to stop refreshing updates)
• "Uninstall" (to uninstall apps)

If one is badly worded, then we can work on improving it.