Split AppNavigation into individual components

[ChristophWurst]

Fixes #25.
Required for nextcloud/mail#1418 in order to add spacers to the navigation (will add the component here once implemented).

Again, this was extracted from Mail and thus follows https://docs.nextcloud.com/server/15/developer_manual/design/content.html.

New structure for apps using our default layout:

<AppContent app-name="mail">
	<template slot="navigation">
		<AppNavigationNew :text="t('mail', 'New message')"
						  buttonId="mail_new_message"
						  buttonClass="icon-add"
						  @click="onNewMessage"/>
		<ul id="accounts-list">
			<AppNavigationItem v-for="item in menu"
							   :key="item.key"
							   :item="item"/>
		</ul>
		<AppNavigationSettings :title="t('mail', 'Settings')">
			<AppSettingsMenu />
		</AppNavigationSettings>
	</template>
	<template slot="content">
		....
	</template>
	<template slot="sidebar">
		....
	</template>
</AppContent>

Breaking change, hence 0.6.

[ChristophWurst]

Not particularly beautiful code, but you can see how I switched Mail to these new components: https://github.com/nextcloud/mail/pull/1456/files (a bit of duplication I yet have to clean up)