Allow exporting animation previews as GIFs

Uses this library to export the GIF:

https://github.com/wernsey/bitmap

I haven't found a way to disable the compression yet,
but this is a good starting point.

Closes #129

app/qml/ui/AnimationPanel.qml

@@ -22,6 +22,8 @@ import QtQuick.Controls 2.12
 import QtQuick.Layouts 1.1
 import QtQuick.Window 2.12
 
+import Qt.labs.platform 1.0 as Platform
+
 import App 1.0
 
 import "." as Ui
@@ -46,6 +48,15 @@ Panel {
         onReadyToSave: root.project.uiState.setValue("animationPanelExpanded", root.expanded)
     }
 
+    Platform.FileDialog {
+        id: saveGifDialog
+        objectName: "saveGifDialog"
+        fileMode: Platform.FileDialog.SaveFile
+        nameFilters: ["GIF files (*.gif)"]
+        defaultSuffix: "gif"
+        onAccepted: project.exportGif(file)
+    }
+
     settingsPopup: AnimationSettingsPopup {
         x: (parent.width - width) / 2
         y: (parent.height - height) / 2
@@ -88,6 +99,14 @@ Panel {
             }
         }
 
+        Ui.VerticalSeparator {
+            padding: 6
+            topPadding: 0
+            bottomPadding: 0
+
+            Layout.fillWidth: true
+        }
+
         RowLayout {
             // We only use one icon from typicons.
             FontLoader {
@@ -119,6 +138,20 @@ Panel {
                 font.family: fontLoader.name
                 onClicked: animationPlayback.loop = checked
             }
+
+            ToolButton {
+                objectName: "exportGifButton"
+                text: "\uf019"
+                font.family: "FontAwesome"
+                focusPolicy: Qt.NoFocus
+
+                ToolTip.text: qsTr("Export the animation preview as a GIF")
+                ToolTip.visible: hovered
+                ToolTip.delay: toolTipDelay
+                ToolTip.timeout: toolTipTimeout
+
+                onClicked: saveGifDialog.open()
+            }
         }
     }
 }

lib/3rdparty/bitmap/LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2017 Werner Stoop
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.