[ui] better UI Components split + improvements

* add WorkspaceView that contains Meshroom main modules (ImageGallery, 2D/3D viewer)
* add Panel component to standardize UI modules layout
* ImageGallery: 
  * add images basename on image delegates
  * add explanatory placeholder when no image has been added to the reconstruction yet

meshroom/ui/qml/filepath.js

@@ -0,0 +1,17 @@
+/* Utility functions to manipulate file paths */
+
+/// Returns the directory name of the given path.
+function dirname(path) {
+    return path.substring(0, path.lastIndexOf('/'))
+}
+
+/// Returns the basename (file.extension) of the given path.
+function basename(path) {
+    return path.substring(path.lastIndexOf('/') + 1, path.length)
+}
+
+/// Return the extension (prefixed by a '.') of the given path.
+function extension(path) {
+    var dot_pos = path.lastIndexOf('.');
+    return dot_pos > -1 ? path.substring(dot_pos, path.length) : ""
+}