GDB plugin: fix icon size, name, theme

- rename the icon to "debug-kategdb"
named after the plugin, this avoid potential clashes
- change the size from 16px to 22px
that is the real size of that png
- install as hicolor and not oxygen
avoid clashes and future specialized oxygen versions, and make it visible
even when not using the oxygen icon theme

R +- -- addons/kate/gdbplugin/hi22-action-debug-kategdb.png [from: addons/kate/gdbplugin/ox16-action-debug.png - 100% similarity]