settings dialog: Remove dark theme setting
MainWindow: Keep the menu button's relief
TweetListEntry: Fix margin-bottom of the avatar

This centers the avatar in scenarios with only a small amount of text.
Add new filter icon

Kindly provided by Allan Day.
user-list-entry: Add some margin

... to make it look more like the TweetListEntrys, especially in the
search page.
TextButton: baseline-align the label
tweet-list-entry: Unify the avatar's horizontal margins
Make filters caseless by default

Fuck it.

closes #187
closes #190
AccountDialog: Move the delete button to the left
account-dialog: Add website entry
InlineMediaDownloader: Also download flickr images

See #222
InlineMediaDownloader: Hide thumbnails for failed downloaded images

If the intermediate website can't be loaded for some reason, just hide
the thumbnail.
Account: Save the website when querying account data
TweetListEntry: Fix the hover button margin_top

Since the grid inside a TweetListEntry now has a margin_top of 6, we
need to adjust the margin_top of the hover buttons when hovered or
adjusted at construction time.
ReplyIndicator: Fix failed frame clock assertion

We only need to add the tick callback in case there are replies.
style: Transparentize the bg of tweet detail buttons
style: Fix hover button padding
compose-window: refine spacing

Make the avatar/textview spacing match that of the TweetListEntry that
is possibly below it (when replying).
Switch more_buttons to go-down-symbolic

open-menu-symbolic only exists in adwaita-icon-theme 3.14
TweetInfoPage: rework some style bits