Changed notes:
1.renamed ProjectPreferencePage to ProjectsPreferencePage
2.added hyperlink that point m2e homepage in ProjectsPreferencePage when projectProvider don't
contain maven selection. When user click the link, we'll call eclipse web
browser to open this page.
3.changed ProjectPreferencePage to ProjectsPreferencePage
plugin.xml file.
4.added code to implement the function set ant as default build type when
maven was uninstalled after user has been selected maven option throuth
ProjectsPreference page.
5.removed unused override method(propertyChange)