Fixes AppImage build

- RawTherapee app icon should be installed in hicolor theme provided in AppImage package
- Fix wrong searching path for SVG/PNG images in rtscalable.h

rtdata/icons/hicolor/index.theme

@@ -0,0 +1,48 @@
+[Icon Theme]
+Name=Hicolor
+Comment=RawTherapee app icon
+Hidden=true
+Directories=16x16/apps,24x24/apps,32x32/apps,48x48/apps,64x64/apps,128x128/apps,256x256/apps,scalable/apps
+
+
+[16x16/apps]
+Size=16
+Context=ApplicationIcons
+Type=Threshold
+
+[24x24/apps]
+Size=24
+Context=ApplicationIcons
+Type=Threshold
+
+[32x32/apps]
+Size=32
+Context=ApplicationIcons
+Type=Threshold
+
+[48x48/apps]
+Size=48
+Context=ApplicationIcons
+Type=Threshold
+
+[64x64/apps]
+Size=64
+Context=ApplicationIcons
+Type=Threshold
+
+[128x128/apps]
+Size=128
+Context=ApplicationIcons
+Type=Threshold
+
+[256x256/apps]
+Size=256
+Context=ApplicationIcons
+Type=Threshold
+
+[scalable/apps]
+MinSize=16
+Size=128
+MaxSize=256
+Context=ApplicationIcons
+Type=Scalable