user-filter-entry: Use a GtkGrid
app: Don't show the window menu twice

Since 3.12, GtkHeaderBar shows the window menu automatically if
gtk_decoration_layout contains a "menu", but since we did the fallback
for quite some time and nobody wants to see the menu twice, we just
remove the menu from the decoration layout.
TweetListEntry: Use GtkGrid
tweet-list-entry: Add margin_top to the text_label
tweet-list-entry: Fix margin_top of first row widgets
Install the corebird icon in more sizes
main-window: Set icon_name

Related to #139