Add dialog allowing you to alter the scenario's graphics sheets from within the scenario editor

- You can copy/paste images into the sheet, or import/export to/from png files

Also:
- Picture controls in the dialog engine have a new "scaled" flag; if set, the picture will be scaled into the provided bounds rather than overflowing. Currently, only full sheets honour the setting.

rsrc/dialogs/graphic-sheets.xml

@@ -0,0 +1,19 @@
+<?xml version='1.0' encoding='UTF-8' standalone='no'?>
+<?xml-stylesheet href="dialog.xsl" type="text/xsl"?>
+<dialog skin='light' defbtn='okay'>
+	<pict type='dlog' num='16' top='6' left='6'/>
+	<text size='large' top='18' left='50' width='250' height='17'>Edit Custom Graphics:</text>
+	<text top='36' left='152' width='80' height='16'>Sheet Number:</text>
+	<text name='num' top='36' left='242' width='40' height='16'/>
+	<pict name='sheet' framed='true' scaled='true' type='full' num='0' top='54' left='12' width='280' height='360'/>
+	<button name='copy' type='regular' key-mod='ctrl' def-key='c' top='54' left='304'>Copy</button>
+	<button name='paste' type='regular' key-mod='ctrl' def-key='v' top='80' left='304'>Paste</button>
+	<button name='open' type='regular' key-mod='ctrl' def-key='o' top='106' left='304'>Import</button>
+	<button name='save' type='regular' key-mod='ctrl' def-key='s' top='132' left='304'>Export</button>
+	<button name='del' type='regular' key-mod='ctrl' def-key='x' top='158' left='304'>Delete</button>
+	<button name='left' type='left' def-key='left' top='426' left='20'/>
+	<button name='right' type='right' def-key='right' top='426' left='85'/>
+	<button name='new' type='regular' key-mod='ctrl' def-key='n' top='426' left='162'>New</button>
+	<button name='cancel' type='regular' def-key='esc' top='426' left='239'>Cancel</button>
+	<button name='okay' type='regular' top='426' left='304'>OK</button>
+</dialog>