Reorganize .gitignore file(s)

Move ignored files in /examples/ and /test/ to their own folders,
i.e. to examples/.gitignore and test/.gitignore, resp.

examples/.gitignore

@@ -0,0 +1,44 @@
+#
+# Files to be ignored by Git (do not commit)
+#
+# Note: *.exe will be ignored by means of ../.gitignore
+#
+browser-simple
+clipboard
+draggable-group
+fltk-versions
+howto-add_fd-and-popen
+howto-browser-with-icons
+howto-drag-and-drop
+howto-draw-an-x
+howto-menu-with-images
+howto-parse-args
+howto-remap-numpad-keyboard-keys
+howto-simple-svg
+howto-text-over-image-button
+menubar-add
+nativefilechooser-simple
+nativefilechooser-simple-app
+OpenGL3-glut-test
+OpenGL3test
+progress-simple
+shapedwindow
+simple-terminal
+SVG_File_Surface
+table-as-container
+table-simple
+table-sort
+table-spreadsheet
+table-spreadsheet-with-keyboard-nav
+table-with-keynav
+table-with-right-column-stretch-fit
+tabs-simple
+textdisplay-with-colors
+texteditor-simple
+texteditor-with-dynamic-colors
+tree-as-container
+tree-custom-draw-items
+tree-custom-sort
+tree-of-tables
+tree-simple
+wizard-simple