Big code refactor

The design was defective and I didn't expect the popularity of the project. After these changes the code will be more mantainable and understandable.

Among the changes we can see:
- A better code structure
- Decoupled button widget from its logic
- More code reuse
- Easier way to add buttons
- Specialized classes

src/capture/tools/exittool.cpp

@@ -0,0 +1,57 @@
+// Copyright 2017 Alejandro Sirgo Rica
+//
+// This file is part of Flameshot.
+//
+//     Flameshot is free software: you can redistribute it and/or modify
+//     it under the terms of the GNU General Public License as published by
+//     the Free Software Foundation, either version 3 of the License, or
+//     (at your option) any later version.
+//
+//     Flameshot is distributed in the hope that it will be useful,
+//     but WITHOUT ANY WARRANTY; without even the implied warranty of
+//     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+//     GNU General Public License for more details.
+//
+//     You should have received a copy of the GNU General Public License
+//     along with Flameshot.  If not, see <http://www.gnu.org/licenses/>.
+
+#include "exittool.h"
+#include <QPainter>
+
+ExitTool::ExitTool(QObject *parent) : CaptureTool(parent) {
+
+}
+
+bool ExitTool::isSelectable() {
+    return false;
+}
+
+QString ExitTool::getIconName() {
+    return "close.png";
+}
+
+QString ExitTool::getName() {
+    return tr("Exit");
+}
+
+QString ExitTool::getDescription() {
+    return tr("Leave the capture screen");
+}
+
+CaptureTool::ToolWorkType ExitTool::getToolType() {
+    return TYPE_WORKER;
+}
+
+void ExitTool::processImage(
+        QPainter &painter,
+        const QVector<QPoint> &points,
+        const QColor &color)
+{
+    Q_UNUSED(painter);
+    Q_UNUSED(points);
+    Q_UNUSED(color);
+}
+
+void ExitTool::onPressed() {
+    Q_EMIT requestAction(REQ_CLOSE_GUI);
+}