Fix status message

Progress count equals the number of files, any files, found, not the
number of image files.
Make all thumbnails strictly 50x50

Otherwise QListView::setUniformItemSize() causes the view to flicker
during inserts because it expects all items are of uniform sizes. And we
really want uniform size because otherwise the thumbnail view would look
like shit.
Enable image actions only if image is set and image view is visible
Move common code to functions
Rename internal column identifier
Save image filepaths in QSet instead of QHash
Trivial optimization
Cleanup includes
Uniform include guard naming
Wrap long line
Merge metadata parsing and reading

Metadata is read if it exists and is newer than the image. Otherwise it
parsed from the image first and then read. Parsing is now hidden from
public.
Rename ImageWidget -> ImageArea
New widget to hold both the image area and the image tool bar
New widget to hold both the thumbnail view and the thumbnail tool bar
Add thumbnail actions to a context menu
Get rid of useless action getters
Remove useless code
Add image actions to a context menu
Update acronym