[Launcher] home button should reset dash department

I agree that the department should reset to the root department when navigating back to the Apps scope; both in the interest of utility as well as in order to be consistent with how we handle search results (which are cleared when navigating away from a scope).

Interestingly, departments are currently not cleared for Apps (as well as other Scopes) when navigating away/too a Scope. However, if a user selects a department and then conducts a search, navigating away/too the scope clears the search AND resets the department (tested in the Amazon and Ebay Scopes). Additionally, opening the department drop-down shows the previously selected department as still being applied.

I would propose that Apps is a special case where we should clear searches and reset departments (including within the department drop-dow) when a user navigates away from the scope.

All other scopes should preserve both the department and (where relevant) filters, but clear searches which are easily accessible by beginning a new search and selecting from the recent search list. I will raise a new bug for this.

when i run unity8 and unity8-dash with qtcreator, i can click dashbutton in launcher to change which scope i see;but when i login unity8 desktop,i click dashbutton then unity8-dash.log show me that

"UbuntuClipboard - Failed to get system clipboard contents via D-Bus. org.freedesktop.DBug.Error.NoReply, Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout xepired, or the network connections was broken"

i have a branch, under these three conditions, clickscope navigation will be reset
1.click homebutton
2.swipe to another scope and success (if your clickscope is the first scope,and you want to swipe from left to right,you will get clickscope, this condition will not reset navigation)
3.gotoscope from manage dash (drop-up from dash bottom)

the BFB now opens the AppDrawer, closing this as invalid