Fix for IZPACK-943

Check the URL for null before creating the ImageIcon. Add a loadIcon()
method to factorize code.
Fix issue IZPACK-1060
Merge pull request #169 from sebastien-christy-neomades/IZPACK-943

Fix for IZPACK-943
Merge pull request #170 from sebastien-christy-neomades/IZPACK-1060

Fix issue IZPACK-1060
[IZPACK-1061] - UserInputPanel: Installer crashes on missing 'set' attribute for RuleInputFields
[IZPACK-1062] - Dynamic variables using checkonce="true" cannot be overwritten by internal variables with the same name
[IZPACK-1059] - AntActionInstallerListener - use graphical InputHandler instead of Ant's DefaultInputHandler in GUI environments
[IZPACK-1059] - AntActionInstallerListener - use graphical InputHandler instead of Ant's DefaultInputHandler in GUI environments
Merge branch 'IZPACK-1059' of github.com:rkrell/izpack into IZPACK-1059

Conflicts:
	izpack-event/src/main/java/com/izforge/izpack/event/AntActionInputHandler.java
Merge pull request #171 from rkrell/IZPACK-1061

[IZPACK-1061] - UserInputPanel: Installer crashes on missing 'set' attribute for RuleInputFields
Merge pull request #172 from rkrell/IZPACK-1062

[IZPACK-1062] - Dynamic variables using checkonce="true" cannot be overwritten by internal variables with the same name
Fix validation of PathInputPanel - chosen path should be validated only on exiting the panel, not on entering the panel
(cherry picked from commit 328825d)
Removed unsused  import
Merge pull request #174 from unb/IZPACK-1015

Fix for IZPACK-1015
Merge pull request #173 from rkrell/IZPACK-1059

[IZPACK-1059] - AntActionInstallerListener - use graphical InputHandler instead of Ant's DefaultInputHandler in GUI environments