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
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