HiDPI support

May be an interesting dev doc

Buttons are displayed small:

Should be fixed on dev branch.

Images are bigger but not buttons:

Same.

Traceback (most recent call last):
  File "src/sakia/main.py", line 120, in <module>
    window = MainWindowController.startup(app)
  File "/home/moul/projects/sakia_git/src/sakia/gui/main_window/controller.py", line 74, in startup
    toolbar = ToolbarController.create(app, navigation)
  File "/home/moul/projects/sakia_git/src/sakia/gui/main_window/toolbar/controller.py", line 50, in create
    view = ToolbarView(None)
  File "/home/moul/projects/sakia_git/src/sakia/gui/main_window/toolbar/view.py", line 64, in __init__
    self.button_network.setSize(self.button_network.size()*dpi_ratio())
AttributeError: 'QPushButton' object has no attribute 'setSize'
ERROR:root:Task was destroyed but it is pending!
task: <Task pending coro=<get_last_version() running at /home/moul/projects/sakia_git/src/sakia/app.py:189>>
sys:1: RuntimeWarning: coroutine 'get_last_version' was never awaited

Much better:

A bit too much big.