Remove "New Calendar" page's .ics file importer button footgun, and clarify the wording

A file imported as a "new calendar" would make it dependent on the file. This feature was confusing everybody and is downright dangerous.

Not only did this complexify the UI, it was also a footgun, so let's eliminate that mis-feature and put the emphasis on the normal expected workflows. This does not prevent anyone from creating normal calendars, and importing events from .ics files into them (which is done through a separate dialog).

Fixes #390 (closed), and clarifies the language throughout this dialog page.

Notes:

• This also drops the gcal-file-chooser-button widget code entirely, as it was not used anywhere else in the codebase.
• Related bugfixing MRs: !417 (merged) and !423 (merged).

Screenshots

Before (46.x)	After this branch