Added new class ResourceAction which centralizes functionality of all
actions related to ResourceItems. This includes loading projects,
samples, presets, plugin-specific presets etc.
Using this new class we can avoid duplicate functionality in
ResourcePreviewer, ResourceBrowser, TrackContainerView, InstrumentTrack
etc.
Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>
