DSL: support sub-glosses with `@`

[soshial]

The structure:

Sub-glosses marked with @ should be openable as separate dictionary entry:

This code:

little
	[m1]1) маленький[/m]
	[m3][*]
	@ little one
	малыш
	@
	[/*][/m]
	[m1]2) немного, мало[/m]
	[m1]3) пустяк, мелочь[/m]
	[m3][*]
	@ little by little
	мало-помалу, постепенно
	@
	[/*][/m]

...should be shown as this:

The block of subglosses is closed whenever a single @ symbol takes the full line.