Remove dead code

All code in the channel manager that is not used anymore in the next-generation version should be removed.

Candidate code paths:

• channel-manager/frontend/src/main/resources/org/onehippo/cms7/channelmanager/templatecomposer/** (i.e. the old template composer ExtJs code)
• channel-manager/frontend-api/src/main/java/org/onehippo/cms7/channelmanager/templatecomposer/** (i.e. the old ToolbarPlugin Wicket-Ext class)
• channel-manager/frontend/js (i.e. the old Angular apps for the menu editor, pages, and page settings)
• channel-manager/frontend/src/main/java/org/onehippo/cms7/channelmanager/channels/ChannelPropertiesWindow.java
• channel-manager/frontend/src/main/java/org/onehippo/cms7/channelmanager/channels/ChannelResourceModel.java
• channel-manager/frontend/src/main/java/org/onehippo/cms7/channelmanager/channels/ChannelStore.java: most methods can be removed excepted ones used by ChannelGridPanel, ChannelIconPanel.