Continuing from #175

Theme & styles

The overall look and feel has a lot of room for improvement:

Main layout

[x] Consistent statusbar/tooltop messages (some widgets only have tooltip or vice versa)
[ ] Draw progress bar on top of status bar instead of reserving extra space at bottom of screen
[ ] Top tabs waste too much space; turn into a sidebar instead?
[ ] Toolbar is visible for all tabs, but the current buttons really only apply to the Photos tab. This should either be merged into the Photos tab, or hidden by default, or include only buttons that apply to multiple tabs.

[x] Add rounded corners to card thumbnails (can't be done in QSS. requires handling in paintEvent)
[x] Clicking text on a taxon or obs info card does not select it (QLabel doesn't allow clicked signal to reach parent widget)
[x] Top of icons sometimes get cut off (weird padding/spacing/margins issue?)
[ ] When loading InfoCardLists, make placeholder the same size as fully loaded card (to get rid of weird effect when loading a very long list)

[x] Newly added history & frequent taxon info cards don't have on-click event bound
[x] Main taxon photo has hover effect + click event when no photo is loaded
[ ] When deselecting iconic taxon filter button, don't select it in taxon view
[x] When a taxon is selected (not as part of an observation), deselect any current observation
[ ] When taxon search is run, switch to search results tab if it's not already displayed
[ ] #278

[x] https://github.com/pyinat/pyinaturalist-convert/issues/94
[x] Disambiguate between 'refresh tags' and 'refresh observations' actions and their toolbar buttons
[x] Add dialog to prompt for username on first startup, before attempting to load observations
[x] Add total count of user observations
[ ] Clicking an observation card multiple times before it loads queues up multiple tasks

[ ] Fix taxon/observation medium photo overlay: resizes to parent widget size instead of photo size