Add Text tool

closes #11
Add an initial version of the text tool.
This commit adds:
- Basic text tool.
- On demand thickness and color update.
- Generalized logic for future widget based tools.

src/tools/marker/markertool.cpp

@@ -86,3 +86,7 @@ void MarkerTool::drawStart(const CaptureContext &context) {
 void MarkerTool::pressed(const CaptureContext &context) {
     Q_UNUSED(context);
 }
+
+void MarkerTool::thicknessChanged(const int th) {
+    m_thickness = th + PADDING_VALUE;
+}