Removes the default tooltips of the buttons to improve clarity

Put the automatically added tooltips back to the default value (empty qSt)
so the user doensn't sees two 'tooltips'

Contributes to #43

resources/qml/Cura.qml

@@ -193,7 +193,7 @@ UM.MainWindow {
 
                 iconSource: UM.Theme.icons.open;
                 style: UM.Backend.progress < 0 ? UM.Theme.styles.open_file_button : UM.Theme.styles.tool_button;
-
+                tooltip: '';
                 anchors {
                     top: parent.top;
                     topMargin: UM.Theme.sizes.window_margin.height;
@@ -232,7 +232,7 @@ UM.MainWindow {
                 iconSource: UM.Theme.icons.viewmode;
 
                 style: UM.Theme.styles.tool_button;
-
+                tooltip: '';
                 menu: Menu {
                     id: viewMenu;
                     Instantiator {

resources/qml/Toolbar.qml

@@ -47,7 +47,6 @@ Item {
             Button {
                 text: model.name;
                 iconSource: UM.Theme.icons[model.icon];
-                tooltip: model.description;
 
                 checkable: true;
                 checked: model.active;