Scalable Setup Dialog

Make the setup dialog resizable and use widgets that dynamically adjust
to the font size set by the user.

Current status of the tabs:
* General: Done
* Performance: Plugins group needs adjustments
* Audio: Individual settings dialogs need adjustments
* MIDI: Not started yet
* Paths: Done

The "Autosave" and "Buffer size" tabs have been slightly redesigned so
that the revert/reset button is next to the slider.

Technical details
------------------
Setting a fixed size has been removed from the setup dialog so that it
can be resized.

The settings widget (`settings_w`) now has a layout to which the sub
dialogs are added. This is done so that the layout sizes are propagated
upwards, i.e. all widgets along the chain now have a layout so that size
hints are determined correctly.

Instances of `TabWidget` are replaced with instances of `QGroupBox`.
Each group box has a layout to which its children, e.g. check boxes, are
added. These group boxes are then added to the layouts of their parents.
Doing so also removes the need to count how many instances have been
added and calculations on where to put the children.

Instances of `LedCheckBox` are replaced with instances of `QCheckBox`.
This is done in the new helper lambda `addCheckBox`. It's very similar
to the previously used `addLedCheckBox` but creates a `QCheckBox`
instead of a `LedCheckBox`. It returns the created `QCheckBox` as a
result. If a layout is provided the created check box is also added to
the layout before it is returned.

The helper lambda `addPathEntry` has been adjusted to create a
`QGroupBox` and to put all its children in a layout.

The helper method `labelWidget` has been adjusted to not set the font
size of the label.

The method `TabBar::addTab` had to be extended with a new default
parameter which controls if the added widget is fixed to the size of
it's parent or not. In the case of the setup dialog we do not want this.
So far the method is only used by the setup dialog and the
`LadspaBrowser` class. So once the `LadspaBrowser` has been made
resizable the default parameter can be removed again and treated as
always being set to false.

Add `QCheckBox` to the `style.css` so that it does not get a green font
due to the palette magic that's done.

data/themes/default/style.css

@@ -3,7 +3,7 @@
  ********************/
 
 /* most foreground text items */
-QLabel, QTreeWidget, QListWidget, QGroupBox, QMenuBar {
+QLabel, QTreeWidget, QListWidget, QGroupBox, QMenuBar, QCheckBox {
 	color: #d1d8e4;
 }