UI improvements

[marySalvi]

• Adds a loading feedback on the filebrowser
• Makes the Image Data Table 'denser', expand across more of the page and increases image preview size (slightly)
• Image lists for the filebrowser are created dynamically based on the space available.

[marySalvi]

@manthey I believe this should also resolve #250. Would you mind checking when you review? Thanks!

[manthey]

The "Collecting Directory Data" message doesn't go away; I was selecting an empty directory for output.

It also didn't go away in a directory that had no sub-directories but had images.

[manthey]

I'm getting outer scroll bars on the page:

[marySalvi]

I'm getting outer scroll bars on the page:

hopefully fixed in d6b0b72. Apologies for the back and forth on this, I am having trouble recreating the issue.

[marySalvi]

The "Collecting Directory Data" message doesn't go away; I was selecting an empty directory for output.

It also didn't go away in a directory that had no sub-directories but had images.

fixed in b283bb6