SideBar: rewritten to use QToolBar instead of KMultiTabBar

Rewrote implementation of class SideBar to use QToolBar instead of
KMultiTabBar. We can style the SideBar now easily via CSS and do not
have to ship 3rd party KDE code with LMMS. Also the QToolBar based
SideBar integrates much better into the according widget style.

Furthermore renamed SideBar related classes and files to match new
coding style.
(cherry picked from commit 822a22ca37)

data/themes/default/style.css

@@ -128,6 +128,14 @@ QToolButton:checked {
 	padding: 3px 0px 0px 3px;
 }
 
+SideBar {
+	background: qlineargradient(x1:0, y1:0, x2:1, y2:0, stop: 0 #66728c, stop: 1.0 #38404e);
+}
+
+SideBar QToolButton {
+	font-size: 12px;
+}
+
 FxMixerView QPushButton, EffectRackView QPushButton, ControllerRackView QPushButton {
 	font-size: 10px;
 }