Handle themes in component-server icon endpoints

Currently, we're managing only one set of icons for a connector, however it is planned to use themes in our icons set.
We need to handle two distincts themes (light/dark) in endpoints providing icons for our environments.

Tasks to do:
module talend-maven-plugin

• update icons validation rules
  • new talend.validation.icons.legacy property
• update svg2png goal to handle themes

module component-server

• handle a new parameter (theme?) on icon resources endpoints to return correct icon
  • New query parameter "theme" added to endpoints: icon, icon/custom, icon/family
• Keep current behavior using the default theme (light) icons set
  • Light theme folder is the default if "theme" parameter not given

component-server configuration parameters:

• talend.component.server.icon.theme.default : Default value: light. Icon default theme (light/dark).
• talend.component.server.icon.theme.legacy : Default value: true. Do we support legacy (not themed) icons. If true, lookup will be done if not themed icon found.
• talend.component.server.icon.theme.support : Default value: true. Do we support icons theme.