This simplifies the build and possibly helps with linking problems
on some platforms. The 'progressive' option allows building
without an intermediate static library except for unit tests.
With a mature history setting up a browser window can be slow, but
if it's setup at creation time it is extremely fast.
The signal prototype of MidoriBrowser::new-window had to change,
all call sites were adjusted.
At this point all downloads run automatically and appear in the
statusbar. The destination folder can be chosen in the preferences.
Download can be cancelled. The context menu works as well.
