Add support for keyboard shortcuts for menu items. Additionally enable a common Ctrl+O for opening new file(s) and Ctrl+U for opening an URI.
Closes#45
Adwaita does not have rounded corners yet, but will have soon. Enforce rounded corners in the app only for Adwaita to avoid bumping min required GTK version.
The "clapper_src" directory name was unusual. This was done to make it work as a widget for other apps. Now that this functionality got removed it can be named simply "src" as recommended by guidelines.