Geany now remembers how many plugins depend on a pluxy. It uses this
information to disable the "Active" checkbox in the PM dialog.

Additionally, the PM dialog displays plugins in a hierarchical manner, so that
sub-plugins are shown next to pluxy. This is espcially handy since it makes
the sub-plugin <-> pluxy relationship really obvious, and it's easier to spot
which plugins need to be disabled before the pluxy can be disabled. This allows
to remove code to re-select the plugin because the row (respective to the
hierarchy level) does not change anymore.