Reword plugins ID, label & descriptions

Proposed resolution

The source plugins from display_builder_views are not following the naming rules expected from Drupal & UI Pattenrs 2 plugins.

Problem/Motivation

Update their labels by following those rules:

• sentence case (because it is the standard way for plugins labels)
• something about the View prefix: by removing it or by putting it in square brackets (see #3525836: [2.0.5] Improve context switching UX)
• follow View Admin UI wording when possible

Proposal:

• ViewHeaderSource: View header >> [View] Header
• ViewPagerSource: View pager >> [View] Pager
• ViewAttachmentAfterSource: View attachment after >> [View] Attachment after
• ViewAttachmentBeforeSource: View attachment before >> [View] Attachment before
• ViewFooterSource: View footer >> [View] Footer
• ViewRowsSource: View rows >> [View] Rows (careful, it may be removed by #3533044: Merge the two ViewRowsSource plugins)
• ViewFeedIconsSource: View Feed Icons >> [View] Feed icons
• ViewMoreSource: View more >> [View] More
• ViewExposedSource: View exposed >> [View] Exposed form

For plugin ID, follow those rules

• Remove _source suffix because Drupal plugin Ids are namespaced by plugin type
• no uppercase characters

Proposals:

• ViewHeaderSource: View_header_source >> view_header
• ViewPagerSource: View_pager_source >> view_pager
• ViewAttachmentAfterSource: View_attachment_after_source >> view_attachment_after
• ViewAttachmentBeforeSource: View_attachment_before_source >> view_attachment_before
• ViewFooterSource: View_footer_source >> view_footer
• ViewRowsSource: View_rows_source >> view_rows
• ViewFeedIconsSource: View_feed_icons_source >> view_feed_icons
• ViewMoreSource: View_more_source >> view_more
• ViewExposedSource: View_exposed_source >> view_exposed