iOS port initial commit

Known Issues
------------
- No printing support.
- No typing support using the OS virtual keyboard.  iOS needs to be signalled
  to bring up the virtual keyboard when the text tool is active.  We also may
  need to do some finagling to make IM work with the virtual keyboard.
- OS language detection doesn't work yet.
- Quitting doesn't close the app.  It just displays a black screen until it is
  force-closed.
- Need to include cross-compilation instructions.

Possible Issues
---------------
- No text display.  This is likely an issue with how pango and related
  libraries were cross-compiled rather than an issue with Tux Paint code.  From
  the error output it appears to be a font rendering issue.
- SVG integration couldn't be tested because RSVG library has not yet be
  cross-compiled successfully.
- Only tested under the iOS Simulator (and not on an actual iOS device yet.)

src/ios.h

@@ -0,0 +1,30 @@
+/*
+  ios.h
+
+  Copyright (c) 2021
+  http://www.tuxpaint.org/
+
+  This program 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 2 of the License, or
+  (at your option) any later version.
+
+  This program 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 this program; if not, write to the Free Software
+  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+  (See COPYING.txt)
+*/
+#ifndef __IOS_H__
+#define __IOS_H__
+
+const char *apple_fontsPath(void);
+const char *apple_preferencesPath(void);
+const char *apple_globalPreferencesPath(void);
+
+
+#endif /* __IOS_H__ */