allow module to ignore filter panel

The `datanames` argument in `modules` can cause the filter panel to not be displayed in a module, which means no filters can be added for that module. However, global filters are applied to **all** modules, including ones do not display the filter panel ("funny1" and "empty") and ones that take no data at all ("empty").


Should we allow an app dev to run an application in which selected modules are exempt from filtering?

My feeling is that data-less modules should show up as "n/a" in the filter manager at least but I believe it should not be done by patching the display. Rather, we should have a way for `teal` to make `FilteredData` inert.

Please discuss.

### modules
```
library(teal.modules.general)
pkgload::load_all("../teal.slice")
pkgload::load_all("../teal")

rm(list = ls())

funny_module <- function (label = "Filter states", datanames = "all") {
checkmate::assert_string(label)
module(
label = label,
datanames = datanames,
ui = function(id, ...) {
ns <- NS(id)
div(
h2("The following filter calls are generated:"),
verbatimTextOutput(ns("filter_states")),
verbatimTextOutput(ns("filter_calls")),
actionButton(ns("reset"), "reset_to_default")
)
},
server = function(input, output, session, data, filter_panel_api) {
checkmate::assert_class(data, "tdata")
observeEvent(input$reset, set_filter_state(filter_panel_api, default_filters))
output$filter_states <- renderPrint({
logger::log_trace("rendering text1")
filter_panel_api %>% get_filter_state()
})
output$filter_calls <- renderText({
logger::log_trace("rendering text2")
attr(data, "code")()
})
}
)
}

dead_module <- function(label = "empty module", datanames = NULL) {
module(label = label,
datanames = datanames,
ui = function(id) {
ns <- NS(id)
tagList(
h4("this is just text")
)
},
server = function(id, filter_panel_api) {
message("hello")
}
)
}
```

### filters
```
default_filters <- teal::teal_slices(
teal_slice("iris", "Sepal.Length"),
teal_slice("iris", "Sepal.Width"),
teal_slice("iris", "Species"),
teal_slice("mtcars", "mpg"),
exclude_varnames = list(
iris = c("Petal.Length"),
mtcars = c("qsec", "drat")
),
module_specific = TRUE,
mapping = list(
funny1 = c("iris Sepal.Length", "iris Sepal.Width", "iris Species"),
funny2 = "iris Species",
global_filters = "mtcars mpg"
)
)
```

# app
```
app <- init(
data = teal_data(
dataset("iris", iris),
dataset("mtcars", mtcars)
),
modules = modules(
tm_data_table(
"table",
variables_selected = list(),
dt_args = list()
),
modules(
label = "tab1",
funny_module("funny1", datanames = NULL),
funny_module("funny2", datanames = "iris"),
dead_module(datanames = NULL)
)
), filter = default_filters
)

runApp(app, launch.browser = TRUE)
```